In the quest of making robots more pervasive in our society, soft technologies are a promising answer to the limitations of traditional robotic systems. State of the art robots are mostly composed of rigid structures and actuators, enabling precise, powerful and specialized motions at the expense of adaptability and safe interaction with the environment and with human operators. Soft technologies shift this paradigm enabling the development of structures, actuators and robots suited for operating in unstructured environments, in proximity to users and for tasks requiring high dexterity or conformability such as manipulation, locomotion, rehabilitation and surgical operations.
Although intrinsic compliance is the core advantage of soft technologies, it can easily become a limiting factor in their usability and versatility. Many tasks require exerting or withstanding substantial forces, which is not trivial for inherently soft systems. For example soft endoscopes can advance through tortuous paths in the human body minimizing pain and damage to the surrounding tissues, but stiffness is mandatory to perform surgical operations. Similarly, manipulation requires compliance for grasping complex shapes as well as rigidity for lifting. Another challenge of soft technologies is the control of a high number of degrees of freedom. The traditional approach consists in using a single actuator for each degree of freedom (DOF), with the drawback of making the system inherently complex and bulky, therefore difficult to miniaturize.
Materials with controllable stiffness have been applied to soft structures in order to selectively tune their load bearing capabilities depending on the task, or to enhance their controllability by selectively stiffening or locking DOF achieving complex configurations and motion patterns with a reduced number of actuators. Some well-known solutions include shape memory polymers (SMP), wax, and electro rheological (ER) and magneto rheological (MR) fluids, which can change stiffness under certain stimuli such as temperature, electric, or magnetic fields.
Accordingly, despite all the advancements in the field of materials with controllable stiffness, in light of the above-described drawbacks and deficiencies of the existing solutions for actuators, strongly improved solutions and methods are desired, with the possibility to switch between soft and rigid states of the actuators.